Commercial sealed lead-acid batteries as manufactured during the 1970's were a reliable source of power, provided long life and were extremely rugged in terms of their construction.
This construction allowed high amperage charge rates. By way of practical example, completely discharged "D" cell batteries when connected to a constant current source may take up to 15 amperes of current producing an internal pressure of 15 psi. Safety vents common to spiral wound battery constructions "vent" at 50psi during extreme abusive situations to prevent the possibility of container rupture. These worthy attributes were recognized and utilized by the commercial industry.
A flat plate sealed lead-acid battery construction was later developed and was dictated by the desire for automated production and consequent significant unit cost reduction. The flat plate design is handled easily by machinery which literally stamps the plates and separators out by the millions and then stacks them. The stacks of plates and separators are sealed into single ply plastic housings after injection with electrolyte. These manufacturing techniques produced sealed lead-acid batteries which are nearly 1/3 the size and weight of a spiral wound battery with equal ampere hour capacity and yield a cost reduction in excess of 50% per unit. Due to these desirable features of low cost, reduced weight and high capacity, the flat plate design is currently dominating the 1980's market place. The major feature which was lost in the flat plate design was the ability to accept unlimited amounts of charge current, this feature only being exhibited by the spiral wound cell construction.
Flat plate cells when charged with an abundant source current will vent, releasing raw materials required for the electro-chemical reaction and thereby decreasing the battery capacity. This fact is immediately recognized by comparison of the pressure required for operation of the safety vent systems in flat plate and spiral wound constructions. Spiral wound cells due to their expensive rugged construction require greater than 50 psi to vent, allowing high charge currents, compared to 1-6 psi necessary for operation of the flat plate vents. To ensure a long life of reliable operation from the flat plate batteries their charge current must be limited to a lower value than spiral wound cells, e.g. 0.4C (0.4.times.ampere hour rating).
To capitalize on the availability of flat plate batteries as a source of cheap Power, it would be advantageous to develop extremely economical and effective circuitry for limitation of charge current.